Nowadays, modern power plants are required not only to be highly efficient but also to have the greatest possible flexibility of operation. This includes, in addition to short start-up times and high load-change speeds, for example also the possibility of equalizing frequency disturbances in the grid. Depending on country-specific requirement profiles for grids and the associated remuneration models, it can therefore be expedient, specifically in the case of gas and steam power plants, in the peak load range, to as quickly as possible make available to the grid additional power by means of the water-steam circuit using an auxiliary firing, also when the gas turbine is already operated at full load.
Using an auxiliary firing, which is generally arranged within the flue gas duct of a waste heat steam generator in the region of the heat exchanger stages which serve as superheaters or intermediate superheaters, increases the heat power transferred to the water-steam circuit, such that the quantity of steam generated and, finally, also the mechanical power released by means of the steam turbine are increased. Thus, the temperatures of the fresh steam and of the intermediate superheater steam increase when the auxiliary firing is switched on. In the reverse case, these both drop again when the auxiliary firing is switched off. The rate of change of the fresh steam temperature or of the intermediate superheater steam temperature is however subject to fatigue-related limits, with regard to the steam turbine as a thick-walled component. While it is still possible, when switching on the auxiliary firing, to effectively counter an impermissibly rapid and large increase in steam temperatures by using an associated injection cooling device, in the reverse case a required reduction in the injection quantity is not infinitely possible. At the latest at the moment at which the injection cooling device is finally closed, the desired temperature of fresh steam or intermediate superheater steam can no longer be maintained by means of a further reduction in the auxiliary firing power. Both steam temperatures begin to slide. Their rate of change is then directly coupled to the rate of change of the auxiliary firing power and can thus, in an unfavourable case, exceed the maximum permissible rates of change of the steam turbine.